Patent GB-791832-A (GB)
Title: Improvements in or relating to traffic control systems and apparatus
IPC: G08G1/082
Filed: 1954-11-30
Abstract: 791,832. Road signals for controlling traffic. EASTERN INDUSTRIES, Inc. Nov. 30, 1954 [Nov. 30, 1953], No. 34722/54. Class 118 (2). A traffic control system wherein remote con. trol of variations of the total length of the signal cycle is obtained while keeping a substantially constant length for a part of the cycle, such as the clearance (yellow) intervals, comprises a local traffic signal controller adapted to be operated cyclically by frequency responsive drive means, a variable frequency power source, a fixed frequency power source, and switch means operated by the said controller during a cycle of operation thereof to connect the variable frequency power source to the drive means during a part of the cycle and to connect the fixed frequency power source to the drive means during another part of the cycle. Fig. 1 shows master control apparatus which provides a variable frequency supply over lines VFL, VFR, and synchronization signals over lines RSL, RSR. Fig. 2 shows two local control imits LCU1 and LCU2, the former being shown in detail. In both Figures the fixed frequency A.C. supply is indicated by the plus and minus signs in circles. The master control apparatus comprises a variable frequency generator GN driven by an A.C. motor whose speed can be set by adjustment of a rheostat PR. The frequency may vary, for example, from 40 to 120 c/s. and is supplied at low voltage and low current, amplifiers being provided in the various units to supply synchronous type motors driving operating cams. Instead of a motor-alternator set an electronic variable frequency generator may be used. The local control unit of Fig. 2 comprises a known drum controller LDC for operating the red, yellow, and green (R, Y, and G) lights of signal S, shown diagrammatically for two lanes A and B. The drum controller LDC is stepped by means of contacts LDS1 closed by preset pins DP on a dial LD1 rotated by a synchronous motor LM. The motor coil LM2-LM3 is fed via the change-over contacts of relays RBY and RAY, connected respectively in the circuits of yellow signal lamps BY and AY. Thus the motor is supplied from the variable frequency lines VFL, VFR (via amplifier LA) when relays RBY or RAY are not operated, but direct from the fixed frequency supply when these relays are operated (i.e. during yellow, clearance, periods). The "go" periods are, therefore, remotely controlled by the master controller, but the yellow periods are constant. The local units are synchronized with the master controller by means of a pin RP on a further dial LDR driven by the motor LM which once in each signal cycle (preferably not during a yellow period) opens contacts LDS3 to disconnect the motor from its supply. The motor LM cannot start again until a normally energized relay LRS releases, and this is controlled over lines RSR, RSL by a cam MCR (Fig. 1) in the master controller. Once per signal cycle contacts MSR are opened and relay LRS released to allow the local motor LM to perform another cycle. The motor MSM driving cam MCR must vary in speed with the variation of the total signal cycle time at a local unit and therefore the motor MSM is driven from the variable frequency supply with a cam MCY to change-over the supply to the fixed frequency source for a period substantially equal to the total yellow clearance signal periods. To ensure that the synchronizing arrangement is effective the motor MSM drives its cams at a slightly slower speed than the local unit motor LM drives the dials LD1, LDR so that pin RP always stops motor LM before relay LRS is released. The phase relationship between the signals of one local controller LCU1 and another, such as LCU2, is determined by the relative positions of the pin RP to the pins DP in each unit. Instead of interrupting the circuit of the motor LM the pin RP could be arranged to close contacts to energize a brake and stop the motor; the contacts of relay LR3 would then be in series with the brake circuit and opened when LRS was de-energized for synchronizing. If there is overlap of green and yellow periods the overlap time is treated as part of the yellow clearance period. In an alternative embodiment (Fig. 3, not shown) relays are connected to be energized with the green signal circuits instead of the yellow, and their contacts switch the motor LM to the variable frequency supply when the relays are energized. In another embodiment (Fig. 4, not shown) a single cam controls both yellow signals over a common circuit and only one relay is provided to switch the supplies to the motor LM. Alternatively a special cam may be provided to close the circuit of a single relay during each green period (Fig. 5, not shown). In yet another embodiment (Fig. 6, not shown), the connections between the motor and the local amplifier or the fixed frequency supply are controlled directly by camoperated contacts in the local controller. Specification 674,590 is referred to.